If you find yourself watching the Olympics - and the swimming in particular - you might start asking yourself 'why is that athlete's body covered in dark circles?'.
It's a question asked every four years and the 2024 Paris Olympics will be no different.
Arguably the greatest swimmer of all time, Michael Phelps, has been photographed after his historic wins with the dark marks (not you, Voldemort) across his back. Why they exist goes back a long, long time.
What are the dark circles?
It is something that back thousands of years. We're talking 1,550 years before Jesus Christ himself was meant to have been born.
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Gaining traction as an ancient Chinese practise, the first records of the dark circles was recorded by the Egyptians more than 3,500 years ago.
To get the circles, you undergo a simply practise called cupping.
What is cupping?
There are two forms of cupping; dry and wet.
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Dry cupping is where a person has a warm cup placed on their skin.
From there, the air in the cup cools and creates a suction around the cup's rim, which is on your skin.
After this, the suction creates something called a haematoma, which is where your blood swells within the tissues.
With wet cupping, a small cut is caused to the surface of the skin and then a cup is put over the top of this.
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The cut will cause you to bleed in to the cup area, and as such carries an infection risk, according to health professionals.
What does cupping do?
Edzard Ernst is an Emeritus Professor of Complementary Medicine at the University of Exeter. Writing for The Conversation, he said that when he had done this to patients in his career, it 'helped ease the pain of most patients'.
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On scientific evidence that cupping works, Ernst explained: "There have been several clinical trials of cupping showing that it can work for pain, but I am not impressed.
"These studies are of very poor quality - many do not have good controls and/or are subject to bias, and some report results which, quite frankly, are too good to be true.
"The procedure is clearly most impressive to the patient. It would be hardly surprising if cupping generated a significant placebo effect."
Ernst compared it to having mild toothache and 'accidentally hitting your thumb with a hammer'.
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"You will find that the counter irritation of the hammer strike made your tooth ache disappear instantly, at least for a while," he said.
Tom Earnshaw